The invention concerns a method for blow molding containers, in which a preform made of a thermoplastic material is subjected to thermal conditioning along a conveyance path in a heating line and then molded into a container in a blow mold by the action of blowing pressure; in which, after the container has been blow molded, a wall thickness is measured at at least one height level of the container; in which an automatic control system is supplied with a preassigned value for the wall thickness as the setpoint value and with the measured wall thickness as the actual value; and in which the automatic control system presets a value of at least one parameter that affects the blowing process as a function of the difference between the setpoint value and the actual value.
The invention also concerns a device for blow molding containers made of a thermoplastic material, which has at least one heating line arranged along a preform conveyance path and at least one blowing station with a blow mold, and in which an automatic control system is used, which is connected with at least one sensor for determining a wall thickness of the container, and in which the automatic control system has at least one final control element for presetting the value of a parameter that affects the blowing process.
In container molding by the action of blowing pressure, preforms made of a thermoplastic material, for example, preforms made of PET (polyethylene terephthalate), are fed to different processing stations within a blow-molding machine. A blow-molding machine of this type typically has a heating system and a blowing system, in which the preform, which has first been brought to a desired temperature, is expanded by biaxial orientation to form a container. The expansion is effected by means of compressed air, which is fed into the preform to be expanded. DE-OS 43 40 291 explains the process-engineering sequence in this type of expansion of the preform. The aforementioned introduction of the pressurized gas comprises both the introduction of compressed gas into the developing container bubble and the introduction of compressed gas into the preform at the beginning of the blowing process.
The basic structure of a blowing station for container molding is described in DE-OS 42 12 583. Possible means of bringing the preforms to the desired temperature are explained DE-OS 23 52 926.
Various handling devices can be used to convey the preforms and the blow-molded containers within the blow-molding device. The use of transport mandrels, onto which the preforms are slipped, has proven especially effective. However, the preforms can also be handled with other supporting devices. Other available designs are grippers for handling the preforms and expanding mandrels, which can be inserted in the mouth region of the preform to support the preform.
The handling of containers with the use of transfer wheels is described, for example, in DE-OS 199 06 438 with the transfer wheel arranged between a blowing wheel and a delivery line.
The above-explained handling of the preforms occurs, for one thing, in so-called two-step processes, in which the preforms are first produced by injection molding and temporarily stored and then later conditioned with respect to their temperature and blown into containers. For another, the preforms can be handled in so-called one-step processes, in which the preforms are first produced by injection molding and allowed to solidify sufficiently and are then immediately suitably conditioned with respect to their temperature and then blow molded.
With respect to the blowing stations that are used, various embodiments are known. In the case of blowing stations that are arranged on rotating transport wheels, book-like opening of the mold supports is often encountered. However, it is also possible to use mold supports that can be moved relative to each other or that are supported in a different way. In stationary blowing stations, which are suitable especially for accommodating several cavities for container molding, plates arranged parallel to one another are typically used as mold supports.
Before a heating operation is carried out, the preforms are typically slipped onto transport mandrels, which either convey the preforms through the entire blow-molding machine or merely revolve within the heating system. In the case of vertical heating of the preforms in such a way that the mouths of the preforms are oriented vertically downward, the preforms are usually placed on a sleeve-like mounting element of the transport mandrel. In the case of suspended heating of the preforms, in which the mouths of the preforms are oriented vertically upward, expanding mandrels are usually inserted into the mouths of the preforms to clamp them tightly.
In carrying out container molding by blow molding, an essential task is to achieve a predetermined material distribution in the container wall. An important parameter for predetermining the material distribution that is obtained is the heat distribution realized in the preforms before the blow molding.
The heat distribution is typically realized in such a way that an even temperature level is produced in a circumferential direction of the preforms, while a temperature profile is produced in a longitudinal direction of the preforms. In addition, a suitable temperature profile through the wall of the preform from the outside to the inside is also predetermined. It can basically be assumed that regions of the preform with a lower temperature lead to thicker wall regions of the blow-molded container, while the warmer regions of the preform are stretched to a greater extent during the blow molding operation and thus lead to thinner wall regions of the blow-molded container.
The temperature of the preforms can be measured with so-called pyrometers. Exact wall thicknesses of the blow-molded containers can be measured with so-called wall thickness sensors, which operate, for example, optically or with the use of sound waves.
Other important parameters for controlling the material distribution in the blow-molded containers are the stretching speed, the assignment with respect to time of the stretching operation to the delivery of compressed gas, and the pressure distribution with respect to time in the expansion of the preform to the container. In particular, controlling the actual blowing pressure has been found to be difficult, because between a control valve for presetting the blowing pressure and the preform to be expanded there lies a flow path with variable passage cross section and throttles that affect the flow, and, in addition, the increase in the volume of the preform during the blow molding of the preform into the container causes a reaction on the developing pressure. On the other hand, the insertion of the stretch rod into the preform leads to a reduction of the available volume. Furthermore, there are relatively complex interactions among the individual parameters, and these interactions affect the actual material distribution that develops in the blow-molded container.
Due to the large number of parameters and interactions among the parameters, instead of actual automatic control, is often only possible to realize control based on the consideration of empirically determined and manually preset adjustments. Practically realized automatic controls typically relate to individual parameters without sufficient consideration having been given to the complexity of the blowing process.